# High Pass Filter (help me)

#### idvc

Joined May 1, 2021
3
Hi, I will design a very simple basic "high pass filter" for my guitar circuit. I want to light touch and shape the sound. I have already equalizer but I want to this inside of guitar circuit.

Here is the filter circuit schematic;

the Cutoff frequency value I want is ~ 397hz.

in this case, I can use R of 40K and C of 0.01uf.

Another option;
I can use R of 20K and C of 0.02uf.

Another option;
I can use R of 10K and C of 0.04uf.

All of them provide about ~ 397hz. What is the difference in between them?

Last edited:

#### LowQCab

Joined Nov 6, 2012
1,082
A Guitar Pick-Up Coil does not have a low enough Impedance to drive ~20K.
You need an Amplifier to Drive a Filter.
The Output will be extremely LOW.
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.
.

#### idvc

Joined May 1, 2021
3
Guitar circuit Already has 250k Volume Potentiometers. I have an guitar amp. My Signal Chain = Guitar-->Guitar Amp.

The filter circuit will add extra 20k ohms, that's true.
in that case, when the Volume potentiometer is completely openned(it is 0 ohm), I will have 0+20k=20k Ohm. Volume is full up.
When The Volume potentiometer is completely clossed(it is 250k), I will have 250k+20k=270k ohm. Volume is completely down.

You're right, the filter circuit added 20kohm but I think it's not important in 0-250k volume potentiometer range. What do you think of?

thank you for reply.

Last edited:

#### idvc

Joined May 1, 2021
3
A Guitar Pick-Up Coil does not have a low enough Impedance to drive ~20K.
You need an Amplifier to Drive a Filter.
The Output will be extremely LOW.
.
.
.
Guitar circuit Already has 250k Volume Potentiometers. I have an guitar amp. My Signal Chain = Guitar-->Guitar Amp.

The filter circuit will add extra 20k ohms, that's true.
in that case, when the Volume potentiometer is completely openned(it is 0 ohm), I will have 0+20k=20k Ohm. Volume is turned up(completely).
When The Volume potentiometer is completely clossed(it is 250k), I will have 250k+20k=270k ohm. Volume is turned down(completely).

You're right, the filter circuit added 20kohm but I think it's not important in 0-250k volume potentiometer range. What do you think of this 20k and the RC value options ?

thank you for reply.

#### LowQCab

Joined Nov 6, 2012
1,082
You need all of the Output-Level that you can get.
I eliminated the Volume and Tone Pots, and replaced them with an FM-Wireless-Transmitter,
and do all Tone changes with a MIDI-Switcher, and a Volume-Pedal.
This provides consistent, repeatable, Presets, with zero-Noise/Hum.

Maybe you should consider running straight into a distortion Pedal and then turn the Gain down.
Most Distortion Pedals have a "High-Pass-Filter" built-in.
.
.
.

#### Ramussons

Joined May 3, 2013
1,059
Hi, I will design a very simple basic "high pass filter" for my guitar circuit. I want to light touch and shape the sound. I have already equalizer but I want to this inside of guitar circuit.

Here is the filter circuit schematic;

View attachment 237243

the Cutoff frequency value I want is ~ 397hz.

in this case, I can use R of 40K and C of 0.01uf.

Another option;
I can use R of 20K and C of 0.02uf.

Another option;
I can use R of 10K and C of 0.04uf.

All of them provide about ~ 397hz. What is the difference in between them?
Thumb Rule:
Choose the RC value so that the impedance is much
less than the input impedance of the next stage and
much higher than the output impedance of the driving stage.

#### Ian0

Joined Aug 7, 2020
3,186
Do you have LTSPICE? If so, you can model a guitar pickup as 6H in series with 6k.
If you add a 20k load resistor, you will also form a low-pass filter with a -3dB point at 690Hz (6H and 26k)
If you go for 2n/200k things improve, and the 200k can be your volume pot.
The resistance of the volume pot won't quite be constant, because the 1M input impedance of the amplifier will be in parallel with part of it: it will vary from 220k to 180k from minimum to maximum, but that's probably the best you will get without adding active circuitry inside the guitar.

#### crutschow

Joined Mar 14, 2008
27,697
the filter circuit added 20kohm but I think it's not important in 0-250k volume potentiometer range.
Not so.
The output impedance of the pot, which varies with its setting, is in series with the filter, so the frequency response will vary with the pot setting.
For example, at half volume the pot equivalent output resistance would be 1/4 * 250KΩ = 62.5kΩ, which would increase the filter corner frequency by a factor of about 4.

#### Ian0

Joined Aug 7, 2020
3,186
I assumed from the original post that the filter should connect directly to the pickup, so this would go before the volume control. In which case, the position of the volume control would make little (but not zero) difference, the difference being caused by the input resistor of the amplifier shunting part of the volume control.